Teeter-restraint device for wind turbines

ABSTRACT

A wind turbine system includes a shaft, a rotor for driving the shaft, and a first fluidic teeter control assembly. The rotor includes a first blade engaged to the shaft by a hub, and has a degree of freedom to pivot relative to the shaft. A first teeter angle is defined between an instantaneous position of the first blade and a time-averaged plane of rotation of the first blade. The first fluidic teeter control assembly is engaged between the rotor and the shaft for providing a first dynamic teeter restraining force as a function of the first teeter angle and a fluidic resistance. The first dynamic restraining force is relatively low when the first teeter angle is within a first teeter operation range, and the first dynamic restraining force is higher when the first teeter angle is outside that range.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. patent application Ser. No.11/899,422 filed Sep. 6, 2007, and titled “TEETER-RESTRAINT DEVICE FORWIND TURBINES”.

BACKGROUND

The present invention relates to wind turbines, and more particularly toteeter control systems for wind turbines.

Wind turbines for converting wind energy to electrical energy typicallycomprise a rotor with one or more blades and a hub. The rotor isattached to, and supported by, a main shaft receiving the rotationalpower from the rotor and transmitting this power to a generator. Themost popular type of large-scale (e.g., multi-megawatt) wind turbinesorient the main shaft in a horizontal direction, thereby making therotational plane of the rotor lie in essentially a vertical direction.

Most contemporary horizontal axis wind turbines use a three-bladedrotor, and fixedly attach the rotor to the main shaft. Accordingly,bending loads on the blades (i.e., loads in a direction substantiallyperpendicular to a plane of rotation of the rotor) are transmitted tothe main shaft. These bending loads originate from uneven winddistribution over the swept area of the rotor, and due to gyroscopicforces associated with the mass of the rotor when the rotor and anacelle are yawed away from the wind direction. The shaft and supportingstructure is, thereby, built according to the weight and strengthrequired to support these loads.

Since the early 1930's, some large-scale wind turbines have employedrotors with one or two blades, with the distinction that the rotor isattached to the shaft through a pin, called a teeter pin, which allowsthe rotor to move perpendicular to a time-averaged rotational plane ofthe rotor, thereby eliminating the transmission of bending loads to themain shaft (when the teetering motion is unconstrained).

An angle between the rotor blades at a given moment and thetime-averaged plane of rotation (essentially a vertical plane) is calledthe teeter angle (β). During normal operation, teeter-angle variation isdesirable: the teeter angle β varies within a certain range which can bedenoted as a standard operating range, and, within that range, changesin response to wind-shear (which produces unequal wind velocity over therotor swept area) and turbulence, and in response to gyroscopic forcesproduced by yawing the rotor into and away from a current winddirection. Due to a lag between load and displacement, maximumteeter-angle values for a two-bladed rotor typically occur when a rotorazimuthal position is essentially horizontal (i.e., parallel to theground). At and around this horizontal rotor azimuthal position, thereis no chance of collision between a blade and the tower (i.e., ablade-tower strike). Only when the rotor is in a vertical azimuthalposition, does a blade pass in the vicinity of the tower. Consequently,the acceptable range of teeter-angle excursions depends on the azimuthalposition of the rotor.

Teetering motion of the rotor reduces bending forces on the rotor thatwould otherwise be present and would cause fatigue in the blades, hub,and main shaft. There are two limits imposed on the teeter angle. Thefirst limit is imposed by the mechanical structures at the rotor tomain-shaft junction. The other, more constraining limit is due toblade-tower collisions. That is, if the teeter angle β increases past acertain value as a blade is passing near the wind-turbine support tower,there is risk of catastrophic blade-tower collision. To avoid this typeof event, most turbines with teetering rotors include a teeter-restraintmechanism that prevents unwanted excursions of the teeter angle.

Two types of teeter restraint mechanisms are found in the prior art.One, which is called the contact type, consists of some flexiblematerial, such as an elastometer or a metal spring, that becomescompressed once the rotor teeter angle exceeds a predetermined amountand contact between the rotor and the teeter restraint mechanism occurs.The restoring force imparted by this type of contact restraint mechanismonto the rotor is quite large, and “impulsive” in nature. Theserestraining loads are undesirable because they promote fatigue andcatastrophic damage, thereby necessitating increased strength and weightin the rotor and nacelle structure. Furthermore, this type of restraintmechanism is independent from the rotor azimuthal position, therefore itprovides unnecessary and damaging restraining force irrespective ofrotor azimuthal position, and hence generates restraining forces even inthe absence of any risk of blade-tower strike.

The other type of known teeter restraint mechanism uses a hydrauliccylinder, regulated by a control system, to provide a non-impulsiveforce restraining teeter motion. With this type of mechanism, teeteringmotion moves the piston within the cylinder, thereby displacinghydraulic fluid into a circuit external to the cylinder. The circuitconnects at least two cylinders, so that the fluid ejected by onecylinder is accepted into the other. Restriction of teeter motion isgenerated by making the hydraulic fluid pass through a constriction, ororifice, located in this circuit. Because the pressure loss across theorifice increases with flow rate, this mechanism provides ateeter-restraint force that is proportional to, and only to, the teeterangular velocity, rather than to the teeter angle itself. This behavioris undesired, because most often, maximal angular velocity occurs as therotor crosses a teeter angle β of zero degrees. Therefore, this secondtype of teeter restraint mechanism places a large, often maximal,restraining force on the rotor when the rotor is at zero teeter angle β,well within the standard operating range, and precisely when thepossibility of tower strike is minimal. This restraining force is cyclic(occurring at every rotor revolution) and produces an unnecessary anddamaging (e.g., fatigue-inducing) load on the rotor and the main shaft.Furthermore, large and beneficial teeter angular velocities also occurduring nacelle-yaw maneuvers, wherein the unconstrained teeter-anglevariation prevents large gyroscopic forces from reaching the main shaft.The second type of mechanism device resists, and fights against theserapid and beneficial teetering motions. In summary, the second type ofmechanism imposes a restraining force on the rotor in conditions whenfree teeter motion is desired, including teeter angles inside thestandard operating range, and teeter-angle excursions during yawmaneuvers, thereby reducing, if not eliminating, the fundamentalbenefits of the teetering rotor design.

In addition, known teeter restraint mechanisms lack means to prevent anyteetering motion at desired times. For example, prior art teeterrestraint mechanisms do not allow teetering motion to be blocked duringstart-up and during parked conditions when the rotor is not rotating.

SUMMARY

A wind turbine system includes a shaft, a rotor for driving the shaft,and a first fluidic teeter control assembly. The rotor includes a firstblade engaged to the shaft by a hub, and has a degree of freedom topivot relative to the shaft. A first teeter angle is defined between aninstantaneous position of the first blade and a time-averaged plane ofrotation of the first blade. The first fluidic teeter control assemblyis engaged between the rotor and the shaft for providing a first dynamicteeter restraining force as a function of the first teeter angle and afluidic resistance. The first dynamic restraining force is relativelylow when the first teeter angle is within a first teeter operationrange, and the first dynamic restraining force is higher when the firstteeter angle is outside that range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation view of a wind turbine system according to thepresent invention.

FIG. 2 is a partial cross-sectional view of a portion of the windturbine system.

FIG. 3 is a cross-sectional view of a first embodiment of a teetercontrol assembly.

FIGS. 4-7 are cross-sectional views of the first embodiment of theteeter control assembly, taken along lines 4-4, 5-5, 6-6 and 7-7,respectively, of FIG. 3.

FIG. 8 is a graph of teeter restraint force versus teeter angle β forthe first embodiment of the teeter control assembly.

FIG. 9 is a cross-sectional view of another embodiment of a teetercontrol assembly.

FIG. 10 is a graph of teeter restraint force versus teeter angle β forthe embodiment of the teeter control assembly of FIG. 9.

FIG. 11 is a partial cross-sectional view of another embodiment of ateeter control assembly.

FIG. 12 is a cross-sectional view of the teeter control assembly of FIG.11 in an engaged position that permits teetering motion.

FIG. 13 is a cross-sectional view of the teeter control assembly ofFIGS. 11 and 12 in a disengaged position that blocks teetering motion.

DETAILED DESCRIPTION

In general, the present invention provides a fluidic teeter controlsystem for a wind turbine that provides a dynamic teeter restraint forcethat varies as a function of teeter angle. This allows relatively lowmagnitude teeter restraint forces to be provided within a first range ofteeter angles, and larger magnitude teeter restraint forces to beprovided outside that first range of teeter angles. This variation ofmagnitude of the teeter restraint force as a function of teeter anglecan be implemented in different ways, such as with varying-depth grooveson an inner diameter wall of a hydraulic piston cylinder, or with ahollow rod that can selectively create a fluid passage across ahydraulic piston in a cylinder. Such teeter restraint forces can beprovided passively, which permits embodiments that lessen wear, fatigueand maintenance concerns. Optionally, an external fluid circuit with avariable resistance valve can be provided to actively control teeterresistance force as a function of a signal from an azimuthal positionsensor, in order to further help reduce a risk of blade-tower strikes.These and other features of various embodiments of the present inventionare explained in greater detail below.

FIG. 1 is an elevation view of a horizontal axis wind turbine system 20that includes a support tower 22 extending vertically from a footing onthe ground 24, a nacelle 26, and a rotor 28. FIG. 2 is a partialcross-sectional view of a portion of the wind turbine system. In theillustrated embodiment shown in FIGS. 1 and 2, the rotor 28 is atwo-blade type with a hub 30 and first and second blades 32 and 34extending from the hub 30. The nacelle 26 (a portion of which is shownin cross-section in FIG. 2) is supported by the support tower 22, andcan move about a yaw axis 36. The rotor 28 is attached to a main shaft38 by a teeter pin 40, and the main shaft 38 is in turn supported by thenacelle 26. In the illustrated embodiment, the main shaft 38 is hollow,although other types of shafts can be used in alternative embodiments. Agenerator (not shown) can be operably connected to the main shaft 38 forgenerating electrical energy. FIG. 1 also shows exemplary atmosphericconditions, including an atmospheric boundary layer 42 and a wind sheargradient 44.

As the rotor 28 rotates, a time-averaged rotational plane 46(essentially a vertical plane) is defined by the sweep of the rotor 28about an axis of rotation 48. The orientation of the rotor 28 in thetime-averaged rotational plane 46 is shown in FIG. 1 in phantom as rotor28′. The teeter pin 40 allows the rotor 28 to move relative to thetime-averaged rotational plane 46. At any given moment, a teeter angle βis defined between the blades 32 and 34 of the rotor 28 and thetime-averaged rotational plane 46. The teeter angle β can vary within amaximum teeter range (i.e., the allowed teeter angle excursion that canbe accommodated by the rotor-to-main-shaft structure), for example plusor minus 8° with respect to the time-averaged rotational plane 46, buttypically varies within a range denoted as a standard operating range,for example, plus or minus 3° with respect to the time-averagedrotational plane 46. Changes in the teeter angle β can occur in responseto turbulence, wind-shear 44 (which produces unequal wind velocity overa rotor-swept area), and gyroscopic forces produced by yawing the rotor28 and nacelle 26 about the yaw axis 36 into and away from a currentwind direction. During normal operation, variation of the teeter angle βis desirable, reducing the transmission of bending loads to the mainshaft 38 (when the teetering motion is unconstrained).

In order to control teetering motion of the rotor 28, one or more teetercontrol assemblies are provided. In the embodiment illustrated in FIGS.1 and 2, a first teeter control assembly 50 and a second teeter controlassembly 52 are connected between the rotor 28 and the main shaft 38.Each teeter control assembly 50 and 52 structurally joints at least oneblade 32 or 34 or hub 30 to a point fixed relative to the main shaft 38.As shown in FIG. 2 with respect to the second teeter control assembly52, each teeter control assembly 50 and 52 is connected to the shaft 38with a teeter restraint pin 54 located at a distance L₂ from the axis ofrotation 48 of the shaft 38. In the illustrated embodiment, the teetercontrol assemblies 50 and 52 are connected to an internal cavity of theshaft 38.

FIG. 3 is a cross-sectional view of a first embodiment of the firstteeter control assembly 50. The second teeter control assembly 52 canhave a similar configuration, and all descriptions of the first teetercontrol assembly 50 can apply to the second teeter control assembly 52.The first teeter control assembly 50 includes a piston 56 having apiston shaft 58 and a piston head 60 movable inside a closed tube 62(e.g., a cylinder) along a piston axis 64. A working fluid, preferablyan essentially incompressible fluid such as conventional hydraulicfluid, is located inside the tube 62. Seals, not shown in the figure,can be used to prevent undesired leakage of the working fluid. A pin(not shown) can connect the piston shaft 58 to a selected point on therotor 28, such as point on the hub 30 that avoids point-loads on theadjacent blade 32. The tube 62 is structurally attached to the nacelle26, relative to the main shaft 38, so that the entire teeter controlassembly 50 rotates with the rotor 28. The attachment of the tube 62 tothe main shaft is through the teeter restraint pin 54 connected to amounting structure extending from the tube 62, whereby forces aretransmitted between the tube 62 and the main shaft 38 while stillallowing rotational motions (i.e., pivotal motions) of the tube 62 withrespect to the shaft 38 and nacelle 26. These rotational motions arekinematical in nature and arise due to the separate location in space(i.e., distance L₂) between the teeter pin 40 and the teeter restraintpin 54 (see FIG. 2).

A change in the teeter angle β of the rotor 28 will cause the piston 56to move along the piston axis 64. Inner diameter walls of the tube 62contain a number of grooves 68 that allow the passage of working fluid(axially with respect to the piston axis 64) allowing passage of workingfluid between a first volume 70 in front of the piston head 60 and asecond volume 72 behind the piston, that is, from one side of the pistonhead 60 to the other. The number of grooves can vary as desired forparticular applications. In one embodiment, the grooves 68 are alignedessentially parallel to the piston axis 64. Typically the grooves 68 aresubstantially equally spaced from each other. The grooves 68 changedepth (i.e., radial depth with respect to the piston axis 64) asfunction of axial location. In particular, the grooves 68 can be deepestnear a axial midpoint of the tube 62, and shallowest at the axialextremities of the tube 62. The grooves 68 can all extend substantiallyan entire interior axial length of the tube 62. In one embodiment,different grooves 68 can have different axial lengths.

FIGS. 4-7 are cross-sectional views of the teeter control assembly 50,taken along lines 4-4, 5-5, 6-6 and 7-7, respectively, of FIG. 3. Thevariation in depth of the grooves 68 is visible in the sequence ofcross-sections shown in FIGS. 4-7 (not all of the grooves 68 arenumbered in FIGS. 4-7). The cross-section of FIG. 4 is taken atapproximately the axial midpoint of the tube 62, where the grooves 68are relatively deep (radially). The cross-sections of FIGS. 5-7 show theradial depth of the grooves 68 becoming progressively lesser toward oneend of the tube 62, as well as the axial length of the grooves 68varying to lessen the total cross-sectional area toward the one end ofthe tube 62.

FIG. 8 is an exemplary graph of teeter restraint force experienced bythe piston 56 versus teeter angle β of the rotor 28. The teeterrestraint force (or restraining force) is the force (impulse) providedby the teeter control system 50 along the piston axis 64 that tends tooppose teetering motion of the rotor 30. When the rotor operates withinthe standard operating range 74, it is most desired to allow the rotor28 to undergo “free-teetering”, that is, teetering that is substantiallyunconstrained by teeter restraining forces from the teeter controlassembly 50. Accordingly, as shown in FIG. 8, restraining force variesprogressively or non-linearly as a function of the teeter angle β. Inthe illustrated embodiment, the teeter restraint force is relatively low(near zero) and constant within a standard operating range 74, that is,a normal range of teeter motion arising in normal operation of thewind-turbine system 20. Outside of the standard operating range 74, theteeter restraining force increases quickly, and approaches relativemaximum values at the greatest teeter angles β within the maximum teeterrange.

For instance, when the piston head 60 is located near the axial midpointof the tube 62, an axial displacement of the piston 56, caused by achange in teeter angle β, will cause working fluid to pass through thegrooves 68 from the first volume 70 on one side of the piston head 60 tothe second volume 72 on the other side of the piston head 60, orvice-versa. Because the grooves 68 are relatively deep at this centralaxial location, resistance encountered by working fluid passing throughthe grooves 68 is small, thereby creating only minimal restraining forceon the piston 56 and allowing essentially free-teeter operation withinthe standard operating range 74.

As the teeter angle β of the rotor 28 exceeds the standard operatingrange 74, the restraint forces raise smoothly and monotonically withteeter-angle β. The teeter control assembly 50 thus allows essentiallyunconstrained, free-teeter motion when the teeter angle β is within thestandard operating range 74, and provides a smoothly increasingrestraining force at increasing teeter angles β outside the standardoperating range 74. When the piston head 60 is located near the axialextremities of the tube 62, the relatively shallow depth of the grooves68 creates a relatively large resistance to the passage of working fluidtherethrough, thereby creating a pressure differential across the pistonhead 60, and, hence, a relatively large restraining force.

The particular relationship between teeter restraint force and teeterangle β can vary as desired for particular application. For instance,the configuration of the grooves 68 influences the relationship betweenteeter restraint force and teeter angle β. However, a suitablerelationship can generally be established as follows. Integrating thearea under a curve plotted on a graph of teeter restraint force versusteeter angle β (e.g., the curve shown in the graph of FIG. 8) willobtain a total force value that can be multiplied by the time over whichthe restraint force is applied to obtain a value of the impulse providedby the teeter restraint assembly 50. The value of the impulse providedby the teeter restraint assembly 50 should be sufficient to stopteetering motion of the rotor 28, which will depend upon the mass andteetering velocity of the rotor 28. Expected teetering velocities of therotor 28 (experimentally known or determined) can be used along with aknown or measured mass of the rotor 28 to anticipate suitablerelationships between teeter restraint force and teeter angle β forparticular applications.

It is well known in the art that the pressure-loss of fluid (e.g., theworking fluid) through an orifice depends both on the orifice size, asdiscussed above in relation to the depth of the grooves 68, and to fluidvelocity. It follows, then, that restraining force provided by theteeter control assembly 50 increases with teeter angular velocity, oncethe teeter-angle exceeds the standard operating range 74. This velocitydependence is desirable, because high teeter angular velocities at highteeter-angles β would otherwise increase the likelihood of a collisionbetween one of the blades 32 and 34 and the support tower 22 (i.e., ablade-tower strike).

All teetering motion, including that in the standard operating range 74,produces working fluid flow within the teeter control assembly 50. Thisflow is unavoidably affected by fluid viscosity, hence is unavoidablyaccompanied by some degree of energy dissipation into heat.Consequently, during operation of the wind turbine system 20, theworking fluid will reach a steady temperature above ambient. The workingfluid temperature is determined by the ratio of energy dissipation toconvective and conductive heat transfer away from the surface of thetube 62 and the piston shaft 58. It should be noted that the teetercontrol assembly 50 is in rotational motion in unison with the rotor 28,hence the teeter control assembly 50 receives an essentially constantflow of relatively cool atmospheric air that transports heat away fromthe cylinder through convection. In order to enhance this heatdissipation, cooling fins (not shown) on the exterior of the tube 62 oran external working fluid cooling circuit (not shown) can be employed tohelp further reduce working fluid temperature.

A thermal condition controller can help regulate working fluidtemperature. For instance, for wind turbine operations in extremecold-weather regions, a working fluid pre-heater 76 (e.g., an electricheater) can be optionally included with the teeter control assembly 50(see FIG. 3) for use while the wind turbine system 20 is stopped tobring the working fluid to operating temperature (the pre-heater 76 isshown only schematically for simplicity). Once the turbine system 20commences operation, the energy dissipation in the working fluidessentially can maintain the operating temperature without the need foradded heat from the pre-heater 76. Alternatively, or in addition, afluid cooling apparatus (e.g., a conventional refrigeration unit) couldbe used like the pre-heater 76 to help control working fluid temperatureby actively removing thermal energy from the working fluid as desired.

The most ideal behavior of the wind turbine system 20 is to regulate theteeter restraining force based on both the teeter angle β of the rotor28, as described above, as well as an azimuthal position of the rotor28. The azimuthal position is the angular orientation of the rotor aboutthe axis of rotation 48. In particular, the two-bladed rotor 28 atazimuthal angles (θ) near zero and 180° (corresponding to a horizontalorientation of the blades 32 and 34 of the rotor 28) has no chance ofblade-tower strike, and the restraining force should be reduced inmagnitude with respect to the restraining force produced when the blades32 and 34 of the rotor 28 are in a vertical position (i.e., azimuthalangles θ of 90° and 270°. Relatively large restraining forces should begenerated by the teeter control assembly 50 only when maximum teeterangles β of the maximum teeter range are approached. To accommodate achange in magnitude of restraining force as function of rotor azimuthalposition, an alternative embodiment of the present invention includes ameans for sensing the azimuthal position of the rotor 28 and a means foradjusting the restraining force resistance in response to the sensedazimuthal angle θ of the rotor 28.

FIG. 9 is a cross-sectional view of an alternative teeter controlassembly 50A. The teeter control assembly 50A can be generally similarto the teeter control assembly 50 described above, but further includesan external fluid circuit 90 connecting to the first volume 70 and thesecond volume at the axial extremities of the tube 62, a variableresistance valve 92 positioned within the external fluid circuit 90, andan azimuthal angle (θ) sensor 94 (shown only schematically forsimplicity). The sensor 94 can be an optical sensor or other type ofconventional sensor suitable for detecting the azimuthal angle θ of therotor 28. The sensor 94 is operably generates an output signal, and thevalve 92 is controlled as a function of that sensor output signal.

Fluid resistance of the valve 92 can be varied as function of theazimuthal angle θ of the rotor 28 as follows. When the azimuthal angle θis near 90° and 270°, corresponding to a vertical position of the blades32 and 34 of the rotor 28, the valve 92 is shut, thereby making theteeter control assembly 50A respond as described above with respect tothe teeter control assembly 50. In particular, with the valve 92 shut,the resistance to motion of the piston head 60 comes solely from thepassage of working fluid through the grooves 68 (not shown in FIG. 9 forclarity) in the tube 62. When the azimuthal angle θ of the rotor 28 isnear zero or 180°, the valve 92 is opened, allowing passage of workingfluid from the first volume 70 in front of the piston head 60 to thesecond volume 72 behind the piston head 60, or vice-versa, and therebyeliminating most of the fluid resistance to motion of the piston 56. Inan alternative embodiment of the teeter control assembly 50A, thegrooves 68 can be omitted and teeter resistance provided solely by theexternal fluid circuit 90. In such an alternative embodiment, a teeterangle (β) sensor (not shown) can be used in conjunction with theazimuthal sensor 94.

FIG. 10 is a graph of teeter restraint force versus teeter angle β forthe embodiment of the teeter control assemblies 50A. As shown in FIG.10, a solid curve illustrates a relatively low magnitude teeterresistance curve that can correspond to restraint force supplied by theteeter restraint system 50A at azimuthal angles θ of the blades 32 and34 of the rotor 28 near zero or 180°. A dashed curve illustratesrelatively high magnitude teeter resistance curve that can correspond torestraint force supplied by the teeter restraint system 50A at azimuthalangles θ of the blades 32 and 34 of the rotor 28 near 90° and 270°,corresponding to substantially vertical position of the blades 32 and 34of the rotor 28 where a risk of blade-tower strike is otherwiseelevated. As shown in FIG. 10, the teeter restraint force for both thesolid and dashed curves is relatively low (near zero) and constantwithin the standard operating range 74.

Another concern with wind turbine operation is a need to block (i.e.,prevent or substantially reduce) teetering motion of the rotor 28 atselected times. For instance, during start-up and parked conditions, itis desired to block teetering motion of the rotor 28.

FIG. 11 is a partial cross-sectional view of an alternative embodimentof a teeter control assembly 50B that includes piston 56B having apiston shaft 58B and a piston head 60B, a tube 62B, and a rod 100 (onlythe tube 62B is shown in cross-section in FIG. 11). FIGS. 12 and 13 arefull cross-sectional views of the teeter control assembly 50B in twodifferent engagement positions. It should be noted that the tube 62B ofteeter control assembly 50B does not include grooves along an innerdiameter wall as with the other embodiments previously discussed.

The rod 100 is arranged coaxially with a piston axis 64B, and includesan opening 102 to an internal cavity 104. The rod 100 can rotate aboutthe piston axis 64B, but is fixed relative to the tube 62B to preventtranslational movement the axial direction. The opening 102 varies inwidth along the piston axis 64B. In the illustrated embodiment, the rod100 is substantially cylindrical in shape, and the opening 102 isrhombic in shape.

The piston shaft 58B has an internal cavity 106 that extends in agenerally axial direction, and the piston head 60B has a central opening108 that adjoins the internal cavity 106. Further, a lateral opening 110is formed in the piston shaft 58B that is in fluid communication withthe internal cavity 106. The lateral opening 110 can have a rectangularshape. As the piston 56B moves within the tube 62B, a portion of the rod100 can pass through the central opening 108 in the piston head 60B andinto the internal cavity 106 in the piston shaft 58B. The piston 56B isrotationally fixed, and can only move linearly along the piston axis 64Bwith respect to the tube 62B.

The rod 100 can be rotated about the piston axis 64B between an engagedposition that permits controlled teetering motion, and a disengagedposition that blocks teetering motion. FIG. 12 is a cross-sectional viewof the teeter control assembly 50B with the rod 100 rotated to anengaged position that permits controlled teetering motion. In theengaged position, the opening 102 in the rod 100 and the lateral opening110 in the piston shaft 58B are aligned, and the assembly 50B allowsworking fluid to flow between the first volume 70 in front of the pistonhead 60B and the second volume 72 behind the piston head 60B. Inparticular, working fluid from the first volume 70 can pass through aportion of the opening 102 and into the internal cavity 104 in the rod100. Working fluid can pass axially through the internal cavity 104,through the opening 108 in the piston head 60B, then back through theopening 102 in the rod and through the lateral opening 110 in the pistonshaft 58B to the second volume 72 (an exemplary fluid flow is designatedby arrow in FIG. 12). FIG. 13 is a cross-sectional view of the teetercontrol assembly 50B with the rod 100 rotated to a disengaged positionthat blocks teetering motion. In the disengaged position, the rod 100 isrotated about the piston axis 64B such that the opening 102 in the rod100 and the lateral opening 110 in the piston shaft 58B are not aligned.In particular, this essentially prevents working fluid from passingbetween the first and second volumes 72, which prevents the piston 56Bfrom moving and thus prevents teetering motion.

The width (or circumferential dimension) of the opening 102 in the rod100 varies along the piston axis 64B, with a maximum width atapproximately its midpoint and less widths toward either end, the fluidresistance to working fluid movement can vary depending on the positionof the piston 56B. In other words, the teetering restraint force canvary as a function of teeter angle β. The teeter control assembly 50Bthus provides an alternative means for varying teeter restraint force asa function of teeter angle β while also providing a way to affirmativelyblock all teetering motion at selected times.

Furthermore, the rod 100 can be rotated such that some overlap betweenthe opening 102 in the rod 100 and the lateral opening 110 in the pistonshaft 58B is provided, but less than full alignment. This also allowsfluid resistance to be controlled by rotation of the rod 100 in additionto control due to axial movement of the piston 56B relative to the rod100.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For instance, features described withrespect to one embodiment, such as azimuthal sensors, variableresistance valves and working fluid thermal condition controllers, canbe readily adapted to other embodiments of the present invention.

1. A wind turbine system comprising: a shaft; a rotor for driving theshaft, the rotor comprising a first blade engaged to the shaft by a hub,wherein the first blade has a degree of freedom to pivot relative to theshaft, and wherein a first teeter angle is defined between aninstantaneous position of the first blade and a time-averaged plane ofrotation of the first blade; and a first fluidic teeter control assemblyengaged between the rotor and the shaft for selectively providing afirst dynamic teeter restraining force as a function of the first teeterangle and a fluidic resistance, wherein the first dynamic restrainingforce is relatively low when the first teeter angle is within a firstteeter operation range, and wherein the first dynamic restraining forceis higher when the first teeter angle is outside the first teeteroperation range.
 2. The system of claim 1, wherein the first fluidicteeter control assembly comprises: a piston tube that defines aninterior surface; a piston movable within the piston tube along a pistonaxis; and a working fluid, wherein the working fluid is displaced as afunction of movement of the piston.
 3. The system of claim 2 and furthercomprising: a groove defined at or near the interior surface of thepiston tube in a generally axial direction with respect to the pistonaxis for allowing the working fluid to pass between a first volumedefined at a first side of the piston and a second volume defined at anopposite second side of the piston.
 4. The system of claim 1 and furthercomprising: a sensor for sensing an azimuthal position of the firstblade; and a control valve for adjusting resistance to displacement ofthe working fluid as a function of the azimuthal position of the firstblade.
 5. The system of claim 4, wherein the control valve increasesresistance to displacement of the working fluid when the azimuthalposition of the first blade is such that the first blade is in closeproximity to a support tower that supports the shaft.
 6. The system ofclaim 1, wherein the first teeter operation range is plus or minus aboutthree degrees with respect to the time-averaged plane of rotation of thefirst blade.
 7. The system of claim 1 wherein the rotor furthercomprises a second blade engaged to the shaft by the hub, wherein thesecond blade has a degree of freedom to pivot relative to the shaft, andwherein a second teeter angle is defined between an instantaneousposition of the second blade and a time-averaged plane of rotation ofthe second blade, the system further comprising: a second fluidic teetercontrol assembly engaged between the rotor and the shaft for providing asecond dynamic teeter restraining force as a function of the secondteeter angle, wherein the second dynamic restraining force is relativelylow when the teeter angle is within a first teeter operation range,wherein the second dynamic restraining force is higher when the teeterangle is outside the first teeter operation range, and wherein the firstand second dynamic restraining forces are independent from each other.8. The system of claim 1, wherein the first fluidic teeter controlassembly comprises: a piston tube; a piston assembly comprising: apiston head movable within the piston tube along a piston axis andhaving a central opening; and an at least partially hollow piston shaftconnected to the piston head and having a piston shaft opening definedthrough a wall of the piston shaft; a rod extending into the piston tubeand having an internal cavity and a rod opening in fluid communicationwith the internal cavity, wherein the rod is arranged coaxially with thepiston axis such that the rod can pass through the central opening inthe piston head and into the piston shaft; and a working fluid, whereinthe working fluid is displaced as a function of movement of the pistonhead, and wherein the rod and the piston assembly are configured toallow alignment of the rod opening, the internal cavity, and the pistonshaft opening such that the working fluid can flow therethrough.
 9. Thesystem of claim 8, wherein a size of the rod opening varies along thepiston axis.
 10. The system of claim 8, wherein the rod opening has arhombic shape.
 11. The system of claim 8, wherein the rod is rotatablefor moving the rod opening and piston shaft opening out of alignment toprevent the working fluid from flowing therebetween.
 12. Ateeter-controlled wind turbine system comprising: a support towerextending in a substantially vertical direction; a shaft supportedrelative to the support tower; a rotor for driving the shaft, the rotorcomprising: a central hub; a first blade engaged to the shaft by thehub, wherein the first blade can pivot relative to the shaft such that afirst teeter angle is defined between an instantaneous position of thefirst blade and a time-averaged plane of rotation of the first blade;and a second blade engaged to the shaft by a hub, wherein the secondblade can pivot relative to the shaft such that a second teeter angle isdefined between an instantaneous position of the second blade and atime-averaged plane of rotation of the second blade; a first fluidicteeter control assembly engaged between the rotor and the nacelle forselectively restraining pivoting motion of the first blade as a functionof the first teeter angle such that pivoting motion of the first bladeis relatively lightly restrained when the first teeter angle is within afirst teeter operation range and pivoting motion of the first blade isrestrained progressively more as the first teeter angle moves outsidethe first teeter operation range; and a second fluidic teeter controlassembly engaged between the rotor and the nacelle for selectivelyproviding a second teeter restraining force as a function of the secondteeter angle such that pivoting motion of the second blade is relativelylightly restrained when the second teeter angle is within the firstteeter operation range and pivoting motion of the second blade isrestrained progressively more as the teeter angle moves outside thefirst teeter operation range, wherein the first and fluidic teetercontrol assemblies operate substantially independent from one another.13. The system of claim 12, wherein the first and second fluidic teetercontrol assemblies each comprise: a piston tube that defines an interiorsurface; a piston movable within the piston tube along a piston axis;and a working fluid, wherein the working fluid is displaced as afunction of movement of the piston.
 14. The system of claim 12 andfurther comprising: a sensor for sensing an azimuthal position of thefirst blade; and a control valve for adjusting resistance todisplacement of the working fluid as a function of the azimuthalposition of the first blade.
 15. The system of claim 14, wherein thecontrol valve increases resistance to displacement of the working fluidwhen the azimuthal position of the first blade is such that the firstblade is in close proximity to the support tower.
 16. The system ofclaim 14, wherein the first teeter operation range for each blade isplus or minus about three degrees with respect to the time-averagedplane of rotation of each blade.
 17. The system of claim 12, wherein thefirst fluidic teeter control assembly comprises: a piston tube; a pistonassembly comprising: a piston head movable within the piston tube alonga piston axis and having a central opening; and an at least partiallyhollow piston shaft connected to the piston head and having a pistonshaft opening defined through a wall of the piston shaft; a rodextending into the piston tube and having a an internal cavity and a rodopening in fluid communication with the internal cavity, wherein the rodis arranged coaxially with the piston axis such that the rod can passthrough the central opening in the piston head and into the pistonshaft; and a working fluid, wherein the working fluid is displaced as afunction of movement of the piston head, and wherein the rod and thepiston assembly are configured to allow alignment of the rod opening,the internal cavity, and the piston shaft opening such that the workingfluid can flow therethrough.
 18. The system of claim 17, wherein a sizeof the rod opening varies along the piston axis.
 19. The system of claim17, wherein the rod opening has a rhombic shape.
 20. The system of claim17, wherein the rod is rotatable for moving the rod opening and pistonshaft opening out of alignment to prevent the working fluid from flowingtherebetween.